In communications networks of today the amount of information communicated over the network has increased. High speed links between network nodes are being used to enable high transmission rates. In optical networks a transceiver arrangement in a network node is arranged to convert electrical data into optical data and transmit the optical data on an optical channel. The optical data is received at a receiving network node and converted back to electrical data. In the field of optical communication modulation of carrier signal in more complex form than the traditional on-off keying is today used to increase the bit rate of data over the optical channel. The modulation of the carrier signal results in that the state of the carrier signal corresponds to a certain input data, whereas a state may in fact contain data corresponding to a number of bits. Hence, by increasing the number of states a carrier signal may take more information, and more bits per second may be transmitted on that carrier. There are different ways to modulate a carrier signal, for example, by modulating the carrier signal in amplitude, phase, frequency, or a combination of parameters. Also, modulating the carrier signal by polarisation of the carrier signal in the polarisation domain may be performed. The idea of using the polarisation domain is not new. It was used in the late 80s and the early 90s with pure polarisation modulation, i.e. no amplitude or phase modulation, and with analogue Stokes receivers, wherein the first wave used non-coherent receivers the second wave coherent receivers.
Today there is a new generation of coherent receivers compared to those developed in the late 80s and discussions of how to mix polarisation modulation with coherent quadrature phase-shift keying (QPSK) systems are ongoing. In this context, not just by using dual polarisation or polarisation multiplexing schemes but using more advanced modulation schemes. WO2009124861 A1 discloses a modulation scheme wherein an optical signal is modulated between a plurality of different states of polarisation and between different phase states. The plurality of polarisation states comprises second states of polarisation located outside the great circle of the Poincaré sphere thereby providing more information per symbol with no or low additional optical signal to noise ration penalty. However, this scheme does focus on the distance between the different states and has a limited maximum of six polarisation states.